In a PFBC plant, the walls of the bed vessel are subjected to very large forces because of the pressure difference appearing across the walls of the bed vessel. In a PFBC power plant with the bed vessel enclosed within a pressure vessel and surrounded by compressed combustion air, a pressure difference arises in operation of the plant between the space in the pressure vessel around the bed vessel and the space inside the bed vessel, because of a pressure drop arising in the fluidized bed. This pressure difference may amount to the order of magnitude of 0.1 MPa (1.0 bar). The walls of the bed vessel may have an area of 200 m.sup.2, and therefore the forces acting on the bed vessel walls are very large, which, having regard to the high temperatures involved, present design problems which are difficult to solve.
The bed vessel is normally of rectangular cross-section and its walls are usually water-cooled. A common construction for the walls consists of panels of vertical coolant tubes which are joined together by intermediate flanges. The tubes can contain feed water to be preheated prior to circulating in the main heat-exchange tubes of the plant. The walls of the bed vessel are not, by themselves capable of withstanding the loads arising because of the pressure difference across the walls. The bed vessel is therefore surrounded by a rigid force-absorbing framework. The bed vessel is supported within this framework by means of force transmitting bars or links. In the case of a cold plant, the framework and the bed vessel walls will have the same temperature but during operation of the plant the walls of the bed vessel assume a temperature close to that of the circulating coolant and the framework assumes a different temperature close to that of the surrounding air. Because of the temperature differences arising between the walls of the bed vessel and the force-absorbing framework, the bed vessel may expand or contract relative to the framework.
The connection between the framework and the bed vessel must be made in such a way that the difference in expansion does not give rise to dangerous stresses in the bed vessel, in the framework, or in the connecting links between the bed vessel and the framework.
German Offenlegungsschrift No. 2 055 803 shows one way of constructing the connection between a conventional boiler and a force-absorbing framework.